Flexographic printing plates are well known for use in letterpress printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, plastic films, aluminium foils etc. Flexographic printing plates can be prepared from photopolymerisable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerisable compositions generally comprise an elastomeric binder, at least one monomer and a photo-initiator. Photosensitive elements generally have a photopolymerisable layer interposed between a support and a coversheet or multi-layer cover element. Upon image-wise exposure to actinic radiation, polymerisation, and hence, insolubilisation of the photopolymerisable layer occurs in the exposed areas. Treatment with a suitable solvent removes the unexposed areas of the photopolymerisable layer leaving a printing relief which can be used for flexographic printing.
Flexographic printing plates used to be made photomechanically wherein the image on a mask film is exposed unto a printing plate but more recently systems were developed to obtain flexographic printing plates wherein the image is directly obtained from image date stored in a computer.
In EP-A-654 150 a flexographic printing element having a incorporated IR ablatable layer and process for making a flexographic printing plate is disclosed.
An opaque IR sensitive layer is image-wise removed by e.g. a semiconductor laser followed by exposure of the photosensitive layer using a back flash UV exposure and a top UV exposure using the image-wise ablated opaque layer as a mask. The plate is then developed to obtain the finished flexographic printing plate. The top layer (black mask), the barrier layer and unexposed parts of the photosensitive layer are removed.
The obtained printing plate can then be mounted onto a press or it is possible to mount it on a sleeve adapted to be quickly mounted onto a printing press. In other systems, such as those for continuous pattern printing, the plate itself has a cylindrical shape for printing the endless image.
Several drawbacks however exist in this working method                As the printing element is prepared using coating method the resulting top layer has a smooth surface. This results in printing results having relatively low ink densities in the centre of solid areas while the edges of these solids result in a larger ink density. This can be adopted by incorporating a surface roughness during fabrication but then the same surface finish is obtained for the whole plate. It is not possible to regulate the surface finish according to image content.        When making the plate the same characteristics are obtained for the whole plate, it is impossible to adapt the plate properties for e.g. printing on a receiving layer consisting of two different materials which would require differential elasticity.        
FIG. 1 illustrates a drawback of prior art flexographic printing plates. The resulting reproduction 3 of a solid 2 on the plate 1 tend to show halo effects 4 as illustrated in the upper side of FIG. 1.
A possible cause for this is that the edges of a solid 2 tend to have a lower printing pressure and thus not squeeze the ink from between the flexographic plate 1 and the printed material as hard as in the middle of the solid areas.
Smaller dots 5 are less susceptible to this effect.                Due to the nature of the exposure the edges of printing areas are not steep, which is also illustrated in FIG. 1. Because the light is spread out in the photosensitive layer the edges of the printing areas forms a slope having an angle α. There is no way to control the angle α or the shape of the slope of these edges.        
Another drawback is that the overall processing time is long. Especially the development and drying time can take a lot of time. In EP-A-654 150 drying times up to two hours are mentioned. Total time for making a plate can take up to 4 hours.
Elaborate steps are needed to get the final result.
The obtained printing plate has the same properties (hardness, roughness) over the entire area and depth. This can result in difficulties in adjusting the printing press settings in order to obtain a good final image without halo effect and with good reproduction of small dots.
Further drawbacks of the state of the art systems is that during fabrication of the plate brushing in presence of a solvent is performed to remove non-printing parts of the plate. During the brushing small size dots tend to wear to a non-printable height or will be removed totally due to the brushing. Although laser imagers are capable to print resolutions up to 10 dots/mm (250 dots per inch) these dots can not be printed due to the loss of small dots in the low density areas due to brushing.
When using digital plates having a carbon layer directly on top of the photopolymerisable layer small dots are even more vulnerable due to the effect of oxygen depletion during development of the plate. This causes exposed dots to be developed smaller than they are exposed and even an insufficient height is obtained to form a printable area.
It is momentarily also not possible to do these steps on a in press system.
In EP-A-641 648 a system is described which gives the possibility to make a flexographic printing plate using an inkjet system.
A photopolymeric ink is jetted onto a substrate to form a positive or negative image which is afterwards cured by UV radiation to form a positive or negative printing plate.
A printing plate is directly obtained without processing and there is no need to development or after processing etc . . .
It is possible to produce printing plates having a desired hardness and thickness.
A drawback of the system described in EP-A-641 648 is that there is no control of the finished product and thus the resulting quality is not ensured.
No control is provided over the overall topography of the produced plate neither the surface finish of the top layer.
Another problem is that during printing of the image the drops of the polymeric ink are not stable and a sharp image is difficult to form. It is impossible to obtain sharp edges as the drops are still mobile and tend to deform.
No attention is given to the problem of stability of reproducing small dots using the plate.
The produced printing plate has the same properties over the entire area of the plate.
The present invention provides a system capable of controlling the plate during production of it. Several properties of the generated flexographic printing plate can be controlled during fabrication. A plate having better properties can be produced.